I know this one. Original 2x4s and other dimension and board lumber were actually the size indicated but were roughsawn and often green. Drying and finishing was left to the carpenter or yard. Eventually lumber yards began to provide the option to face this lumber for the carpenter, but of course the wood removed meant you got smaller dimensions.
When mills started producing faced lumber themselves, they adopted the smaller dimensions of yard or craftsman post-processed lumber. And then since they had already abandoned the real dimensions, they figured what would it hurt to take another 1/32 out here and there? Since every mill had its own definition of reasonable allowances, things got messy once lumber started to be shipped out nationally. Eventually, retailers, builders and the Departments of Commerce and Defense forced the lumber industry to adopt national standards. But since they let the industry do it themselves, the standards naturally codified whatever dimensions resulted in the largest profit they felt they could get away with.
See this 1964 report by the US Forest Service for all the exciting details: http://www.fpl.fs.fed.us/documnts/misc/miscpub_6409.pdf . I mean its got several appearances by Herbert Hoover! And the green vs. dry measurement battle of the West's Douglas Fir producers against the Southern Pine producers!
Can anyone tell I used to be a reference librarian?
